Officers moved into the Friday Street Car Pack on Saturday evening and discovered the group of people who were 'mingling' between the parked vehicles.
No one attending the gathering was wearing a mask.
Under tier three guidance, people should not meet socially with any friends and family you do not live with or who is not a member of your support bubble.
Chorley Police said the registrations of all vehicles at the car park on Saturday evening had been noted by the officers attending.
The group were issued with a warning that if they returned to the car park they would be fined for a breach of tier three restrictions.
A spokesperson said: "None of the people were wearing masks and the attendees were blatantly breaching Tier three restrictions.
"All vehicle registrations have been taken via the use of body worn video and noted down by officers.
"All attendees have been warned that we are compiling a list of vehicles and should they return to the location then fixed penalty notices will be issued for breaching Tier three restrictions."
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